Both Heitkamp and Donnelly were elected in 2012, giving their constituents a full six years to digest their new position. By that point we may be counting down the final Republican senators who don't support gay marriage.

"In speaking with North Dakotans from every corner of our great state, and much personal reflection, I have concluded the federal government should no longer discriminate against people who want to make lifelong, loving commitments to each other or interfere in personal, private, and intimate relationships," Heitkamp said in her statement.

"With the recent Supreme Court arguments and accompanying public discussion of same-sex marriage, I have been thinking about my past positions and votes," Donnelly wrote. "In doing so, I have concluded that the right thing to do is to support marriage equality for all."

The two are only the latest in a wave of Democrats and some Republicans who have come out for gay marriage over the last month. On Thursday, Sen. Bill Nelson (D-FL) joined the equality movement, writing that "If we are endowed by our Creator with rights, then why shouldn't those be attainable by Gays and Lesbians?" On Tuesday, Sens. Tom Carper (D-DE) and Mark Kirk (R-IL) announced they backed same sex marriage.

Some twelve senators have come out for gay marriage since Sen. Rob Portman (R-OH) became the first Republican member to do so, citing his gay son as the inspiration for his move.

That leaves the Democratic caucus down to the Final Four when it comes to marriage equality. At this point, it's possible — maybe even likely — that one of them will go down in history as the last Democratic senator ever to oppose gay marriage.

They could be tougher nuts to crack, however. Sens. Mary Landrieu (D-LA) and Mark Pryor (D-AR) are facing challenging re-election campaigns in two of the most socially conservative states in the nation. Sen. Joe Manchin (D-WV) has worked extremely hard to differentiate himself from national Democrats and might actually see some upside in being the lone holdout. Sen. Tim Johnson (D-SD) is retiring, which gives him more leeway to follow his heart in any direction he chooses.

The Republican side of the aisle might be more fertile ground for marriage equality advocates at this point. Sen. Lisa Murkowski (R-AK) recently said she was "evolving" on the issue and Sen. Susan Collins (R-ME) is likely to face intense pressure to change her stance in the run up to the Supreme Court's expected June decision on a pair of marriage equality cases given her state's liberal lean.

But the recent exodus from the anti-equality camp makes clear that momentum is decisively, overwhelmingly, on the side of same sex marriage proponents, who now hold a majority of the Senate. It's a question of when, not if, the rest of the members will get in line.